If you’re one of the Marines battling a race of insectoid conquerors through the dusty streets of L.A. in “Battle: Los Angeles,” you hack it apart with a combat knife — and look for any way to kill it.

By the time the body is found, the Marines have discovered that the creatures are remarkably tough to kill; like horror movie villains, they get shot again and again, but keep rising from the dead. So, with the aid of a conveniently available civilian veterinarian, the troops begin a hasty dissection, searching for the hidden weakness in an indestructible enemy.

As its squishy insides are spilled, the alien groans, but never says a word. The wide-eyed vet wonders at the lack of defined organs or internal structure. “Does it have some sort of cognitive mechanism? Anything?” she asks.

The answer is no — not just for the disemboweled alien, but, sadly, for the movie itself. Unlike the best alien invasion films, “Battle: Los Angeles” doesn’t have any real ideas or anything to say. Like its organless invaders, it’s basically an empty shell.

For decades, Hollywood has pitted pitiful humans against powerful aliens. From “War of the Worlds” to “E.T.” to “Independence Day” to recent entries like “Skyline,” “Invasion,” and “V,” Tinseltown has introduced viewers to a galaxy’s worth of otherworldly visitors: Some are friendly, most are not; some are slimy, others are scaly, and some look a lot like us — or at least seem to. But when it comes to the movies themselves, the best of the bunch have always had one thing in common: a decidedly terrestrial message or metaphor through which to understand the little green men.

This is true all the way back to the prototype of the alien invasion blockbuster, “War of the Worlds.” H.G. Wells’ 1898 source novel was a subtle critique of British imperialism at the height of its overseas empire. Making no attempt to communicate with the indigenous earthlings, the book’s invaders overwhelm humanity with superior technology, before finally succumbing to an unseen force to which the native population is mostly immune: the common cold.

“Worlds” wasn’t preachy, but it wasn’t empty entertainment either, and it drew its power and much of its lasting appeal from its ability to channel then-contemporary political concerns into a memorable fantasy.

Fifty years after Wells’ book, Hollywood finally started bringing cinematic alien invaders to the planet Earth. And as in Wells’ story, Hollywood’s invasions often paralleled terrestrial politics.

In the years after World War II, Cold War paranoia drove Hollywood’s first wave of alien invasions. Films like the original “Invasion of the Body Snatchers” and “It Came From Outer Space” depicted aliens taking over not by military might, but by converting fellow humans into zombified others.

But moviegoers got muscular invasion films, too: “Earth Versus the Flying Saucers” and George Pal’s adaptation of “War of the Worlds” both featured grand attacks on major landmarks, and were marked by the toppled-city imagery that reflected the recent world wars.

In the 1970s and ‘80s, Hollywood’s alien attackers gave way to a kinder, gentler succession of extraterrestrials. Films like “Close Encounters of the Third Kind,” “The Man Who Fell to Earth,” “Cocoon,” “E.T.,” and “Starman” introduced aliens both innocent and powerful. Sometimes stranded among us, they were misunderstood, and attacked for it. These features worked as fish-out-of-water stories, but also as cautionary tales about America’s growing technological prowess — and the parallel dangers of self-destruction.

In 1996, “Independence Day” kicked off the modern era of invasion blockbusters. It’s a memorable but essentially shallow film with a hastily grafted on environmental message. A better take on the same sort of big-budget invasion spectacle came in Steven Spielberg’s 2005 remake of “War of the Worlds.” The movie’s visual splash was bolstered by Spielberg’s decision to depict the film’s invasion with spooky 9/11 imagery. The movie drew its considerable emotional power from its repurposing of those tragic events.

Measured by historical standards for the genre, Hollywood’s recent mini-wave of alien invasions seems decidedly lackluster. 2007’s “The Invasion,” the third big-screen variant on “Invasion of the Body Snatchers,” was a chase picture set in Washington, D.C. It traipsed all over the city, but somehow managed to avoid bumping into any sort of meaningful political ideas.

Last year’s “Skyline,” meanwhile, was an artless computer graphics showcase — made, according to its effects-guru directors, expressly to prove that a large-scale invasion flick could be shot on the cheap.

Of the recent entries, only ABC’s ongoing series, “V,” fares well — in large part because of its willingness to engage, and sometimes undermine, the hope-and-change politics of the Obama era. It’s often clunky, but the show’s vision of outwardly peaceful aliens who seem to offer peace and prosperity, yet are not at all what they seem, frequently makes it a pleasingly subversive diversion.

Don’t get me wrong: Simple-minded human-alien slugfests can be good, escapist fun on their own terms. But for any sustained engagement, aliens present a problem for dramatists and storytellers: No one really knows what they’d be like — and if they were truly alien, we might not be able to understand anything about them at all.

That helps explain why Hollywood’s alien invasions work better when they’re metaphors for something a little more human: Crafted with any skill, those metaphors provide a framework through which to interpret the unknowable. And they can turn dumb blockbuster fun into something approaching real art, which, at its best, reflects and comments on the society that created it. To put it another way, Hollywood’s alien invasions work best when they help us understand ourselves.